The battle for control of the Grameen Bank, the pioneering Bangladeshi microfinance institution, has reached a new intensity after its government-appointed chairman unilaterally declared that its founder and managing director, the Nobel prize-winning economist Muhammad Yunus had stepped down because he was too old. The bank, however, said Yunus remained in his his post.

Yunus founded the bank almost 30 years ago and has been lauded as one of the world's most innovative and effective campaigners against poverty. But in recent months the government of prime minister Sheikh Hasina has mounted an increasingly aggressive campaign against Yunus, allegedly to take control of what would be a big political asset.

The grounds for the claimed dismissal was a finding by the country's central bank that Yunus, who is 70, had broken local laws demanding mandatory retirement at 60.

Grameen lends small sums of money to more than 8 million families and is hugely popular.

Yunus, an outspoken critic of the corruption endemic in his homeland, angered senior politicians in 2007 by briefly floating a political party. A long legal battle is now expected.

"I find it rather strange that after so many months, they have suddenly woken up to this irregularity," said Dr M Zahir, a supreme court lawyer in Dhaka. "The central bank is the guardian of all banks. They are supposed to be active. What were they doing for so many months and years?"

In recent weeks, a powerful group of international figures, led by Mary Robinson, the former president of Ireland, have backed Yunus's leadership.

The Friends of Grameen denounced the attempt to "destabilise" Yunus in a statement in Paris. According to Grameen's constitution, the government has a 25% stake and appoints three members of the board out of a total of 12. The other nine members are elected by the bank's clients.

The campaign against Yunus has been building for months. It has included a summons to appear in court to face charges that social business ventures resulted in the sale of contaminated yoghurt and accusations from Hasina that Yunus was "sucking blood from the poor".

Yunus has also had to answer claims in a television documentary last year that Grameen transferred funds from Norway's aid agency in the 1990s from one legal entity to another for tax purposes. The Norwegian government said an inquiry had found no evidence of misuse of money.

Last month the Bangladeshi finance minister, Abul Maal Abdul Muhith, called Yunus a "man of high standing and respect" but "now old" and said the bank needed "closer regulation".

The attacks on Yunus come at a time when microlending – once hailed as a model that would change the lives of hundreds of millions in the developing world – faces increasing hostility from politicians across the region.

In India, politicians have accused bankers of profiteering from the poor and, in some places, have banned further lending or recovery of debts. In the southern state of Andhra Pradesh, aggressive selling by scores of unregulated microfinance firms has pushed huge numbers of already desperately poor farmers deeply into debt.

Experts say distinctions need to be made between "for profit" and "not for profit" microfinance institutions such as Grameen.